Processing aids (process auxiliaries) are needed only in relatively small amounts, but are important additives without which some synthetic polymers would be difficult to process and certain shaping processes would be impossible to carry out. They are used with most thermoplastic synthetic polymers and thermoset polymers (curable moulding compositions), and have a decisive role in raising the output performance of production machinery, improving the quality of finished products, and permitting the use of new processing technologies.
Processing aids are used to improve flow behaviour and flowability of synthetic polymers, to improve homogeneity and stability of filled and unfilled synthetic polymer mixtures and synthetic polymer melts, to improve surface quality, and to reduce the adhesion of the melts or mixtures to machinery components, and for the resultant improvement in mould-release properties.
The requirements for suitable processing aids when used together with synthetic polymers are generally the following:                There must be no impairment of the underlying physical properties of the polymers, but there has to be the desired effect on the application-related properties of the finished components. Examples of these properties are surface smoothness, gloss, transparency, printability, weldability, tack, and also slip behaviour and blocking behaviour.        There must be no increase in the adhesion of synthetic polymer compositions and synthetic polymer melts to machinery components or tooling components, or in the resultant decomposition of the polymer. It is desirable that processing aids have anti-adhesive properties.        It should be possible to optimise the rheological behaviour of the synthetic polymers—from powder feed through to melt—and of the compounded synthetic polymer material, in order to obtain homogeneous plasticized synthetic polymer melts and/or homogeneous compounded synthetic polymer materials.        
An example of an overview of processing aids and their possible applications is found in Hans Batzer, Polymere Werkstoffe, Band II—Technologie 1 [Polymeric materials, volume II—Technology 1], Georg Thieme Verlag Stuttgart, 1984, pp. 328 et seq., and Gächter/Müller, Kunststoff-Additive [Plastics additives], 3rd edition, Carl Hanser-Verlag, 1989, pp. 441–502.
TABLE 1Processing aids and their uses:Class of CompoundUsesHydrocarbons, e.g.,Lubricants and releaseparaffin oil - polyethyleneagents for unplasticizedwaxesand plasticized PVC, PS, PAand UP 0.1–1.0%, e.g.,:higher surface gloss, e.g.,in PEAmount used up to 5%Alcohols, e.g., palmityl,Extrusion, injectionstearyl, tallow fattymoulding, calander - ofalcoholunplasticized and plasti-cized PVC(Amount: 0.5–1.0%)Improvement in flowability,internal lubricant action,good compatibility,transparencyKetones, e.g., stearoneBlown PO films as slipagent, reduced adhesion offilms to machinery rollersCarboxylic acids, e.g.,Processing of unplasticizedstearic acids, oxidizedand plasticized PVC, PS,polyethylene waxesCA, MF, and compoundedrubber materialsMetal salts of carboxylicProcessing of plasticizedacids, e.g., Ca, ZnPVC, PO, PS, ABS, PF andstearatesMF. A different lubricantaction depending on type,effect on melting behaviourof PVC, often used ascostabilizersCarboxamides, e.g., fattySlip agents; antiblockingacid monoamides, fatty acidagent for POdiamidesLubricant, slip agent inPVC, PS, PO processingCarboxylic esters, e.g.,Lubricant for unplasticizedfatty acid esters, estersand plasticized PVC, PS, PAof polycarboxylic acidsand MF, processing aid forextrusion, calandering,blow moulding and vacuummoulding of PVC
The abbreviations used herein, e.g., in the table above have the following meanings:                PVC=Polyvinyl chloride        PS=Polystyrene        CA=Cellulose acetate        MF=Melamine-formaldehyde resin        PO=Polyolefin        ABS=Acrylonitrile-butadiene-styrene copolymer        PF=Phenol-formaldehyde resin        PA=Polyamide        UP=Unsaturated polyester resin        PE=Polyethylene        
Although a wide variety of processing aids are known, problems continue to arise in the use of these substances in synthetic polymer compositions. For example, it is known that some processing aids migrate at room temperature to the surface of finished components and thus impair their appearance, or lead to the release of undesired residues (emissions) due to their volatility. For example, stearic acid can deposit on the surface of plasticized PVC as an undesired white deposit (blooming).
Refined hydrocarbons, which are used as lubricants in the production of impact-modified PVC, can also leave a residue (deposit) on the polymer surface at room temperature and give the synthetic polymer a greasy and dull appearance. When metal salts are used, e.g., zinc stearate, particularly in hot-curing synthetic polymer compositions, problems arise since there can be residues (emissions) on the finished parts due to the metal salts. These metal salts also cause adhesion problems when the finished parts are painted. These adhesion problems necessitate the use of additional steps, e.g., alkali treatment (power wash), to improve surface-coatability of the finished component. This additional operation is undesired and is also hazardous to the environment.
Other problems (effects) can arise between processing aids and certain components. For example, exceeding the solubility limit of the processing aid in the polymer can lead to “plate-out” when using certain dyes, pigments, stabilizers and fillers. Often, this phenomenon (plate-out) occurs when short-chain fatty esters are used.
Use of processing aids in polyolefin thermoplastics can result in undesirable interactions with light stabilizers, other stabilizers and/or antioxidants. This may result in disruption of the processing in the machinery and/or loss of stability in the finished products.
U.S. Pat. Nos. 4,210,571 and 4,210,572 disclose coupling agents for the treatment of fillers for hot-curing resins based on carboxylic ester derivatives of mono-, di- and trihydroxy fatty acid esters of mono- or polyhydric alcohols having from 1 to 4 carbon atoms, and use of these. These coupling agents create adhesion between the filler and the resin.
European Patent Application No. 0 22 977 B1 (EP '977) (corresponding to German Patent No. DE 36 50 587 T2, U.S. Pat. No. 4,622,354) discloses the preparation of a curable, phase-stabilized polyester moulding composition.
A problem with many curable moulding compositions, e.g., with polyester resins, is that the individual constituents of the polyester resin are mutually immiscible. Therefore, during processing the components tend to separate into individual phases (phase separation). According EP '977, the use of fatty acids having from 5 to 28 carbon atoms reduces the tendency towards phase separation. A disadvantage here is that the fatty acids disclosed can react with the MgO additive and prevent the additive from performing its function (adjustment of final processing viscosity).
U.S. Pat. No. 4,172,059 discloses a curable moulding composition prepared from an unsaturated polyester resin or from a terminally unsaturated vinyl ester resin, and also discloses mixtures of the two components with an olefinically unsaturated copolymerizable monomer, a thickener, and a filler, and at least 0.4% by weight of a viscosity-lowering processing aid. The specification discloses that the viscosity-lowering processing aid is an aliphatic monocarboxylic acid having at least 6 carbon atoms in the chain. Again, a disadvantage of this system is that the monocarboxylic acids disclosed can react with the additive (MgO).
Although there are a wide variety of processing aids on the market, there are no substances which favourably affect the surface quality of synthetic polymer compositions, advantageously affect the flow properties of synthetic polymer compositions (both during preparation and during processing), and also improve parameters such as surface-coatability and homogeneity. Thus, one aspect of the invention is to identify processing aids that do not exhibit the weaknesses listed.
Another aspect of the invention is to identify processing aids that fulfil the stringent environmental requirements such as, having minimum migration tendency and emission tendency. Thus, there is a need for processing aids that have low migration and or release of undesired residues (emissions) due to their volatility (fogging, blooming, etc) in the final application (products).
Surprisingly, the objects were achieved by using partial esters of carboxylic acids.